1. Technical Field
The present invention relates to a centrifugal fan used for blowing air.
2. Description of the Related Art
There are known centrifugal fans that inhale air in an axial direction parallel to a rotation axis and exhale air in a radial direction perpendicular to the axial direction. The centrifugal fans include an impeller having a plurality of blades circumferentially arranged around the rotation axis and accommodated in a housing.
Japanese Unexamined Patent Publication No. 7-111756 (hereinafter, referred to as JP '756) describes air inlets provided on both sides of the housing in the axial direction of the centrifugal fan (i.e., inhaling air from both sides of the housing). This technique increases a flow rate in the centrifugal fan. In the centrifugal fan of JP '756, a cup-shaped portion having an opening is arranged in the housing to be open downward. A magnet for producing a magnetic field is arranged on an inner side face. An armature is provided inside the magnet. A circuit board for supplying a driving current to the armature is arranged below the opening of the cup-shaped portion. The circuit board is arranged inside an outer periphery of the cup portion in the radial direction. The air inlet facing the opening of the cup-shaped portion is formed at a position that is opposed to the blades of the impeller and is therefore radially outside the circuit board.
Japanese Unexamined Patent Publication No. 2002-291194 (hereinafter, referred to as JP '194) discloses a centrifugal fan in which a part of a stator of a motor is formed by the circuit board. The air intake is provided through the circuit board, thereby allowing inhaling of air from a stator-side of the centrifugal fan.
Japanese Unexamined Patent Publication No. 2001-241395 and Japanese Unexamined Utility-Model Publication No. 62-115772 (hereinafter, referred to as JP '395 and JP '772, respectively) disclose a centrifugal fan in which electronic parts for driving the centrifugal fan and the circuit board are arranged radially outside the blades of the impeller.
In recent years, demands for size reduction of centrifugal fans used in electronic devices have increased with size reduction of the electronic devices. There is, however, a technical limitation on size reduction of electronic parts to be mounted on the circuit board. That is, even if the size of the cup-shaped portion is reduced, it is difficult to make the circuit board smaller than an outer profile of the cup-shaped portion, which approximately corresponds to an area of the opening of the cup-shaped portion, when the circuit board and the cup-shaped portion are seen in the axial direction. Therefore, it is difficult to arrange the entire circuit board radially inside the cup-shaped portion so as not to interfere with the air inlet opposed to the blades of the impeller as proposed in JP '756.
In order to prevent interference of the circuit board with the air inlet, the circuit board may be arranged radially outside the blades of the impeller as disclosed in JP '395 and JP '772. In this case, however, the size of the impeller has to be largely reduced in order to ensure a space for the circuit board radially outside of the blades. This causes various problems, for example, degradation of the air-blowing performance of the centrifugal fan, a cumbersome and complicated work for electrically connecting the reduced-sized armature to the circuit board (it is hard to electrically connect the armature to the circuit board in a case where the armature is too small), and increase in likelihood of occurrence of disconnection between the armature and the circuit board.
Moreover, the circuit board may form a part of the stator of the motor in the housing as disclosed in JP '194. In this case, however, disconnection can easily occur, because an external force directly acts on the circuit board when the fan is carried or attached to an electronic device. Furthermore, when electronic parts on the circuit board are arranged within the housing, an airflow generated in the housing is disturbed by the electronic parts, thus causing noises.